This invention relates to an oscillating circuit, especially an oscillating circuit which is suitable to generate an oscillating output having a stable oscillating frequency phase-locked to a reference signal of a constant frequency.
In an apparatus dealing with a video signal, it is necessary to use high-frequency pulse signals having predetermined phases related to a video signal period, for example, a horizontal scanning period. The high-frequency pulse signals are used as a sampling pulse or a clock pulse of an apparatus for converting the video signal to a digital signal, a delay circuit including a charge coupled device (CCD) for the video signal, and so on.
In the prior art, a phase locked loop (hereinafter PLL) having a voltage controlled oscillator (hereinafter, VCO) has been popularly used. Basically in this loop, an output of the VCO is divided, the divided output is phase-compared with a reference signal, and the VCO is controlled by an error signal thereof, so that a stable signal having higher frequency than the frequency of the reference signal could be obtained. Generally in a video apparatus, as a frequency of the reference signal, a horizontal scanning frequency (f.sub.H) is used and as an oscillating frequency of the VCO, a frequency equal to or lower than 910 times f.sub.H is used. Since f.sub.H of the NTSC color television system is 15,734.246 Hz, 910 times this frequency becomes about 14.3 MHz. As it is possible for digital circuit elements in the market to operate adequately at such a frequency, the above PLL circuit has been used.
In the above art, however, since it is necessary to use a divider which divides the output of the VCO into the frequency of the reference signal, there are some problems. That is, the circuit scale becomes large and a noise generated at the VCO is superimposed on the video signal.